Required selection on inoput type from another field

No problem.

The problem is that you are mis using the intent of a multiple choice field type.   Instead you should be selecting records from a relationship.  

You should make a table of car choices where the Key field is the text field of the car choice.  Then load up that table with all the chopices.

Then maker a relationship based on your existing field.

 You will find that the record picker experience will be vastly improved for the users as they will be able to type small parts of the car name and have the choices filer almost instantly..